Method and an arrangement for measuring and controlling electrode positions in electric furnaces and the like



March 26, 1968- KJQLSETH ET AL 3,375,318

METHOD AND AN ARRANGEMENT FOR MEASURING AND CONTROLLING ELECTRODE POSITIONS IN ELECTRIC FURNACES AND THE LIKE.

Filed Oct. 22, 1964 2 Sheets-Sheet 1 7 I F/G.

dR dH INVENTORS. OVE KJQLSETH 8: SVEN HARRY WILLNERS ATTORNEYS March 26, 1968 o. KJQLSETH ET AL 3,375,318

METHOD AND AN ARRANGEMENT FOR MEASURING AND CONTROLLING ELECTRODE POSITIONS IN ELECTRIC FURNACES AND THE LIKE} Filed Oct. 22, 1964 2 Sheets-$heet 2 I B V I I m I! I2 I3 INVENTORS OVE KJQLSETH 8 SVEN HARRY WHLNERS a mwwmm ATTORNEKS United States atent Fice 3,375,318 METHOD AND AN ARRANGEMENT FOR MEASUR- ING AND CONTROLLING ELECTRODE POSI- TIONS IN ELECTRIC FURNACES AND THE LIKE Ove Kjalseth, Gslo, Norway, and Sven Harry Willners, Lidingo, Sweden, assignors to Elektrolremisk A/S, Oslo, Norway, a company of Norway Filed Oct. 22, 1964, Ser. No. 405,726 Claims priority, application Sweden, Oct. 24, 1963, 11,683/ 63 6 Claims. (Cl. 1313) ABSTRACT OF THE DESCLOSURE A furnace electrode is displaced by an arbitrary differential in height and the effect of such displacement upon a dependent electrical parameter, such as the resistance, is measured. The derivative of the change in resistance with respect to the displacement in height is formed and such derivative alone, or multiplied by the resistance value comprising one or the other of the limits of the change in resistance or preferably the average of the limits, is correlated to the actual height of the submerged tip of the electrode above a pool of molten material in the furnace. In this way, the position of the electrode tip can be accurately determined even though it is not directly observable during operation of the furnace.

The present invention relates to the control of electrothermic processes and, more particularly, to the control of such processes in apparatus of the type where the heat is generated by an electric current passing through a re sistant medium, in which a number of electrodes are submerged.

The invention is primarily concerned with electric reduction furnaces in which the resistive medium is formed by the charge introduced in the furnace. However, it should be understood that the invention is generally applicable in connection with all electrothermic processes based on the generation of heat in a resistive fluid. Thus, it could be utilized also in e.g. electric boilers. However, since the advantages afforded by the invention are most apparent in electric reduction furnaces, the invention will be described hereinbelow in connection with such a furnace. For the sake of simplicity reference will be made to a furnace in which the electric current passes through one or more electrodes, thence through the charge and finally back to the current source through the melt located below the charge. It should, however, be observed that the invention could be applied irrespective of whether the current pattern is as just described or if the current passes from one electrode through the charge and then back through another electrode. Similaly, neither the number of electrodes nor the type of current, direct or alternating current, has any influence on the applicability of the invention.

If the electric curent passes in the manner above indicated from one electrode through the charge and down into the melt, a reaction zone is formed in the charge, i.e. between the electrode point and the surface of the melt. While the exact shape of that zone is not entirely known. it has been assumed here that it has an annular crosssection and is located between the sides of two imaginary concentric, frustrated cones, the bases of which coincide with the surface of the melt and apexes of which are located inside the electrode a distance above the submerged point thereof. On the other hand, it is well-known that the operational economy of the electrothermic process and the composition and properties of the resulting melt are optimized if the electric heat power, generated within each volume unit of the reaction zone, exceeds a certain minimum value and preferably is held substantially constant. These ideal conditions are difficult to achieve because as the reaction goes on, a successive melting of the charge occurs and correspondingly the level of the underlying melt also rises successively. This level rising continues until the melt is discharged whereupon the same thing is repeated up to the next discharge and so on. It follows, therefore, that the distance between the top surface of the melt and the lower point of the electrode will vary considerably. Due to the fact that the reaction zone is between the electrode point and the melt, these variations will cause corresponding changes in the height of the reaction zone. Because of the conical shape thereof, the volume of the reaction zone will be substantially proportional to the third power of the height, i.e. a certain change of the height causes a relatively much greater change of the volume. Not only the volume of the reaction zone but also the impedance conditions therein have a decisive influence on the specific power development. When the reaction volume is increased the height is likewise increased, involving a greater resistance but, at the same time, the cross-sectional area is augmented which corresponds to a resistance decrease. Due to the special and not entirely investigated shape of the reaction volume it is, however, diflicult to define any general relationship between on the one hand the resistance and, on the other, the volume or the height, respectively, of the reaction zone. Accurate positioning of the submerged tip of the electrode, to compensate for the foregoing unavoidable variations and thereby maintain ideal or close to idea] operating conditions within the furnace, has been difficult to accomplish due to the fact that the submerged lower end of the electrode is not directly observable during operation of the furnace.

Various attempts have been made to solve the problem above accounted for. A solution which is entirely reliable but very primitive and time-consuming resides in the use of a metal rod which is introduced through an aperture in the furnace cover for the purpose of making it touch the electrode point. Due to the presence of the solid charge it generally is not feasible to determine the establishment of such a contact by relying on touch only for which reason the rod must be withdrawn and re-introduced several times. Not until the end of the withdrawn rod is glowing can it be concluded that it has been in contact with the electrode point. Thereafter, determination of the inclination of the rod in that position makes it possible to determine the level of the electrode point. According to more recent and, above all, more convenient methods the measuring of the location of the electrode point is instead based on the measurement of some electric variable, e.g. resistance, current intensity, power or phase lag. The measured values are then transformed into pulses triggering a displacement of the electrode upwards or downwards so that the desired optimum furnace operating condition is maintained as much as possible. However, there does not exist any simple relationship between any of the electric variables and the level of the electrode point and, therefore, these prior art measuring and controlling methods can only give results which are at best very rough approximations. A further limitation is that the change of the level of an electrode point made on the basis of eg current or power control causes a change in the current conditions also at the other electrodes so that their positions have to be adjusted as well. This adjustment in turn causes a change of the current conditions at the first electrode and so on. Accordingly, such a control method is not stable but produces repeated oscillations.

The main object of this invention is to provide a method and an arrangement which eliminates the disadvantages and limitations described above and permits exact determination of the position of the point of an electrode submerged in a resistive medium, e.g. the charge of a furnace. The invention is based on the concept that even if the level of the electrode point is not available for direct observation or measuring, this is not true concerning changes in the position of that level. It is therefore possible to determine such changes in the vertical position of the electrode and the resulting variations in one or more electric variables andto create therefrom a number of comparison which with great accuracy can be utilized for measuring or controlling the position of the electrode point. Stated more particularly, this number of comparison at the minimum comprises the derivative of the variation in the measured electrical parameter with respect to the displacement in height of the electrode point level (relative to an arbitrary reference level). Beside the abovementioned parameters others may be used, e.g. the capacitance between the electrode and the melt or between the electrode and some other portion of the furnace. The sole essential requirement is that use is made of the variations of the electrode point level and of the variations to which one or more electric parameters are subjected in consequence of the electrode displacement. Calculations and experiments have shown that the simplest and most suitable expedient is to utilize the resistance variation, i.e. the derivative of change in resistance in respect of the electrode position is determined. According to a preferred embodiment of the invention there is then formed a number of comparision comprising the product of that derivative and of the measured absolute resistance value. Preferably, the last-mentioned valve should be the average value of the two resistance values which are measured in succession in order to determine the limits of resistance variation caused by the arbitrary displacement of the electrode. But it is also possible instead to use one of those two limit values although the accuracy is then lesser.

The invention will now be described in greater detail, reference being made to the accompanying drawings, in which:

FIGURE 1 is a diagram illustrating how variations in the composition of the charge inside a reduction furnace may affect the resistance of the charge;

FIGURE 2 is a diagram illustrating the absolute value of the resistance and also its derivative in respect of ,the electrode level. Both curves have been drawn as a function of the electrode level; and

FIGURE 3 is a block diagram showing the principal design of an arrangement for carrying out the method according to this invention.

Curves I and II in FIGURE 1 show resistance R as a function of H, i.e. the distance between the electrode point and the top surface of the melt, curve I corresponding to a higher resistivity than curve II. The area marked with inclined lines corresponds to a range of furnace operating conditions in which the variations experienced in reaction zone volume and temperature variations are tolerable from a practical point of view. The figure also shows that if the power control of the furnace is regulated in such a manner that the resistance is maintained constant within the limits of the resistivity variations defined by curves I and II, it will be possible to keep the resistance at a selected constant for very different H values, ie, for very different sizes of the reaction zone volume. However, it has been explained herein above that such volume variations are very disadvantageous. For these reasons, regulation of the furnace to a constant resistance in conventional manner cannot be relied upon to maintain the submerged end of the electrode at proper height to produce an optimum reaction zone volume in the furnace.

FIGURE 2 shows that curve dR/dH has a steeper slope than curve R or, stated in other words, that a certain change of H causes a greater change of dR/tiH than of R. It follows therefore that for this added reason a control based on the derivative in accordance with the invention is more sensitive, and moreover feasible as a practical matter, than a simple resistance control.

Turning now to FIGURE 3 reference numeral 1 therein relates to a furnace having a cover 2 traversed by three electrodes 3 partially submerged in a charge 4. Below the charge is a melt 5. H indicates the distance between the lower electrode point and the top surface of the melt. As has been mentioned above, in carrying out the method according to the invention the level of the electrode point is changed and that change, dB, is determined by measuring the corresponding displacement in the position of the electrode outside the furnace. The corresponding feeding device has been shown for one of the electrodes only but it is to be understood that in practice all electrodes are equipped with similar control units. Each such control unit consists of an electric motor 6 which over a shaft 7 drives a pulley or a winch 8 from which there is suspended a rod or wire 9 supporting the electrode 3.

The measured value dH is fed into block 10 which is also supplied with information concerning the voltage V and the current intensity I at the controlled electrode. Since suitable electric equipment for such transmission of information can be designed in many difierent ways and as several suitable designs are known, it is not necessary in this context to describe their nature. To understand the invention it is only necessary to realize that on the basis of the values supplied to block 10 it is possible to determine the impedance, or the resistance R, and its derivative with respect to height H. An electric signal representing that derivative dR/dH is then fed to block 11. If desired and preferably, block 11 is also supplied with information concerning the absolute resistance value R. That supply takes place if an electric switch 14 is in its closed position. From block 11 there is fed to block 12 a pulse which consequently represents either dR/dH only or the product To block 12 is also fed current from some suitable current source 13 for motor 6. Block 12 accordingly contains means, e.g. a switch .or a phase shift apparatus, which in response to the character of the control pulse delivered from block 11 causes motor 6 to rotate in the one or in the opposite direction whereby electrode 3 is raised or lowered. The arrangement may be calibrated by means .of curves or tables compiled empirically. It is also clear that the arrangement may operate fully automatically or semi-automatically and continuously or discontinuously. Of course, it is also possible to arrange instiuments indicating and/ or registrating the electrode positions. For the formation of the product of the absolute resistance value and the derivative it is feasible qiute simply to use an instrument, the indication of which is proportional to the product of two input voltages e.g. a

watt-meter. In certain cases it may be preferable for each electrode to utilize two motors 6 one of which provides the displacement on which the measuring is based while the other carries out the control based on the result of that measuring. Alternatively one motor only may be used which is operable at difierent speeds or has means for variation of its gear ratio so that the measuring movement can take place at a slower rate than the control movement. The time interval between two successive measurements may correspond to a constant magnitude i5 dH This gives a control which is optimized as far as distance H is concerned. Should it instead be desired to carry out the control in such a manner that the temperature at the electrode point is kept as high as possible without exceeding the critical value above which certain disturbances arise, there could alternatively be used a number of comparisons containing the product dR'I Also in several other respects the invention may be varied and modified within the scope of the appended claims.

What we claim is:

1. In a method for determining the location of an electrode end in an electrical apparatus comprising a resistive medium in which at least one such axially displaceable electrode is partially submerged and which is electrothermically heated by the passage of electric current there through, the steps of axially displacing said electrode to vary the position of its submerged end, measuring the magnitude of said displacement and the magnitude of the corresponding variation caused thereby in one of the electrical parameters which are dependent upon the electrode position, forming a number of comparison which includes the derivative of said parameter variation with respect to said electrode displacement, and correlating said number of comparison to the actual position of said submerged electrode end.

2. A method as in claim 1 which includes the step of converting said number of comparison into an electrical signal.

3. In a method for determining the location of an electrode end in an electrical apparatus comprising a resistive medium in which at least one such axially displaceable electrodes is partially submerged and which is electrothermically heated by the passage of electric current therethrough, the steps of axially displacing said electrode to vary the position of its submerged end, measuring the magnitude of said displacement and the magnitude of the corresponding variation caused thereby in one of the electrical parameters which are dependent upon the electrode position, and forming a number of comparison which includes the derivative of said parameter variation with respect to said electrode displacement multiplied by the value of the resistance of said resistive medium, and correlating said number of comparison to the actual position of said submerged electrode end.

4. In a method for determining the location of an electrode end in an electrical apparatus comprising a resistive medium in which at least one such axially displaceable electrode is partially submerged and which is electrothermically heated by the passage of electric current therethrough, the steps of axially displacing said electrode to vary the position of its submerged end, measuring the magnitude of said displacement and the magnitude of the corresponding variation caused thereby in one of the electrical parameters which are dependent upon the electrode position, forming a number of comparison which includes the derivative of said parameter variation with respect to said electrode displacement multiplied by the value of the reciprocal of the current intensity squared, and correlating said number of comparison to the actual position of said submerged electrode end.

5. A control device for determining the position of the submerged end of an axially displaceable electrode in a furnace comprising means for axially displacing said electrode to vary the position thereof, means for measuring the magnitude of said displacement, means for measuring the magnitude of the corresponding variations thereby caused in an electrical parameter which is dependent upon the electrode position, means for forming the derivative of said parameter variation with respect to said electrode displacement, and means for converting said derivative into a signal representing the actual position of the submerged electrode end.

6. A device as in claim 5 which further includes means for controlling the position of said electrode in response to said signal.

BERNARD A. GILHEANY, Primary Examiner. R. N. ENVALL, 111., Assistant Examiner. 

